This invention relates to a thermoelectric module. In more detail the invention relates to a nondegrading lead telluride thermoelectric module with low contact resistance junctions.
The principle of thermoelectric conversion has been known for many years but until recent years has found little other application than in temperature measurement and control. Recently, however, development of power systems for use at remote locations--such as in space--has become necessary and thermoelectric conversion has been utilized for this purpose.
Desirable properties for thermoelectric materials include (1) proper free-electron generation characteristics, i.e., the number of free electrons must be sufficient to overcome internal electrical resistance, yet not so great as to introduce scattering effects, (2) low thermal conductivity so that heat cannot flow through the material without performing some useful work, (3) ability to operate at high temperatures, and (3) various additional property requirements depending on the specific application for the material.
Because of (1) and (2) above, semiconductor materials are more efficient thermoelectric materials than ordinary metals. One material that has found practical application is lead telluride (PbTe), a binary compound of lead and tellurium capable of operation at temperatures up to 1100.degree.-1200.degree. F. By addition of an impurity (a process referred to as doping) the lead telluride can be made to operate as an n- or p-thermoelectric material. Although lead telluride has been used more widely than any other thermoelectric material up to this date, thermoelectric modules made from lead telluride have not performed to the levels theoretically available from the material. An additional problem to that of achieving high-level performance is that of performance degradation during operation of the module. Postoperative examinations of elements from modules have identified the p-type elements as being the primary source of the trouble; specifically the resistance increases in the hot zone of the p-type leg of the thermocouple. This degradation of the module substantially reduces module life from that theoretically attainable.
A substantial effort has, of course, been made to identify the cause of and to suggest means for reducing the degradation of lead telluride thermoelectric modules. We feel that we have identified this cause and have suggested means whereby degradation of such modules can be substantially reduced, thereby providing longer life for the modules.